Internet Engineering Task Force (IETF) J. Levine Request for Comments: 8463 Taughannock Networks Updates: 6376 September 2018 Category: Standards Track ISSN: 2070-1721
A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM)
This document adds a new signing algorithm, Ed25519-SHA256, to "DomainKeys Identified Mail (DKIM) Signatures" (RFC 6376). DKIM verifiers are required to implement this algorithm.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
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DKIM [RFC6376] signs email messages by creating hashes of selected message header fields and body and signing the header hash with a digital signature. Message recipients fetch the signature verification key from the DNS. The defining documents specify a single signing algorithm, RSA [RFC3447] (which has since been obsoleted by [RFC8017]).
This document adds a new, stronger signing algorithm, Edwards-Curve Digital Signature Algorithm, using the Curve25519 curve (Ed25519), which has much shorter keys than RSA for similar levels of security.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376].
The Ed25519-SHA256 signing algorithm computes a message hash as defined in Section 3 of [RFC6376] using SHA-256 [FIPS-180-4-2015] as the hash-alg. It signs the hash with the PureEdDSA variant Ed25519, as defined in RFC 8032, Section 5.1 [RFC8032]. Example keys and signatures in Appendix A are based on the test vectors in RFC 8032, Section 7.1 [RFC8032].
The DNS record for the verification public key has a "k=ed25519" tag to indicate that the key is an Ed25519 rather than an RSA key.
This is an additional DKIM signature algorithm added to Section 3.3 of [RFC6376] as envisioned in Section 3.3.4 of that document.
Note: since Ed25519 public keys are 256 bits long, the base64-encoded key is only 44 octets, so DNS key record data will generally fit in a single 255-byte TXT string and work even with DNS provisioning software that doesn't handle multistring TXT records.
The syntax of DKIM key tags in Section 3.6.1 of [RFC6376] is updated by adding this rule to the existing rule for key-k-tag-type:
key-k-tag-type =/ "ed25519"
The p= value in the key record is the Ed25519 public key encoded in base64. Since the key is 256 bits long, the base64 text is 44 octets long. See Appendix A.2 for a sample key record using the public key in [RFC8032], Section 7.1, Test 1.
For backward compatibility, signers can add multiple signatures that use old and new signing algorithms. Since there can only be a single key record in the DNS for each selector, the signatures have to use different selectors, although they can use the same d= and i= identifiers.
The example message in Appendix A has two signatures with the same d= and i= identifiers but different a= algorithms and s= selectors.
[FIPS-180-4-2015] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, DOI 10.6028/NIST.FIPS.180-4, August 2015, <http://nvlpubs.nist.gov/nistpubs/FIPS/ NIST.FIPS.180-4.pdf>.
The text in each line of the message starts at the first position except for the continuation lines on the DKIM-Signature header fields, which start with a single space. A blank line follows the "Joe." line.
DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=football.example.com; firstname.lastname@example.org; q=dns/txt; s=brisbane; t=1528637909; h=from : to : subject : date : message-id : from : subject : date; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=; b=/gCrinpcQOoIfuHNQIbq4pgh9kyIK3AQUdt9OdqQehSwhEIug4D11Bus Fa3bT3FY5OsU7ZbnKELq+eXdp1Q1Dw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=football.example.com; email@example.com; q=dns/txt; s=test; t=1528637909; h=from : to : subject : date : message-id : from : subject : date; bh=2jUSOH9NhtVGCQWNr9BrIAPreKQjO6Sn7XIkfJVOzv8=; b=F45dVWDfMbQDGHJFlXUNB2HKfbCeLRyhDXgFpEL8GwpsRe0IeIixNTe3 DhCVlUrSjV4BwcVcOF6+FF3Zo9Rpo1tFOeS9mPYQTnGdaSGsgeefOsk2Jz dA+L10TeYt9BgDfQNZtKdN1WO//KgIqXP7OdEFE4LjFYNcUxZQ4FADY+8= From: Joe SixPack <firstname.lastname@example.org> To: Suzie Q <email@example.com> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com>
We lost the game. Are you hungry yet?
John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 United States of America